Electric Plant and Depreciation - Additional Information (Details) $ in Thousands		12 Months Ended
	Dec. 30, 2013 USD ($) Substations mi	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Public Utility Property Plant And Equipment [Line Items]
Electric plant, net includes plant acquisition adjustments		$ 28,600	$ 28,700
Payments to acquire electric plant		$ 239,157	$ 210,791	$ 390,283
Electric Plant Held For Future Use
Public Utility Property Plant And Equipment [Line Items]
Electric plant, transmission lines | mi	66
Electric Plant Held For Future Use | Stanton
Public Utility Property Plant And Equipment [Line Items]
Number of substations | Substations	2
Payments to acquire electric plant	$ 37,100